This invention relates generally to the field of telephony, and more particularly to an improved overvoltage arrestor of a type used in conjunction with a building entrance terminal to provide protection to subscriber equipment. Devices of this general type are known in the art, and the invention lies in specific constructional details which permit the device to offer an improved degree of protection.
A typical protective arrestor of this type is illustrated in the U.S. Pat. No. 3,703,665, to Yearance et al. granted Nov. 21, 1972. The disclosed protector includes an elongated threaded housing, the outer portion of which is provided two engaging means to facilitate threaded engagement with a corresponding socket in the station protector terminal. A coil spring within the housing urges a cage-like retaining member outwardly of the housing and into contact with a grounded contact on the station protector terminal. In that type of similar module employing heat sensitive means, a solder pellet is provided within the cage which melts under heat and flows through an opening in one of the carbon electrodes to reach the other electrode and effectively short the arcing gap by providing another path to a source of ground potential.
As indicated in the Yearance patent, the problem of electrode erosion with continued use is a long standing problem. It is known to configure the opposed arcing surfaces of the electrodes which are made of carbon to improve the life span thereof. Unfortunately, the erosion ultimately becomes great enough that the gap between the electrodes is excessive, and arcing no longer occurs at the previously predetermined breakdown voltage.
It has been previously considered that the great bulk of carbon erosion takes place during surges of substantial excessive current. What has not been appreciated is that relatively small surges also cause erosion, and are much more common. It has been determined, for example, that sneak currents in the order of 100 milliamps continued for a period of two minutes is sufficient to enlarge the gap between the electrodes to a point where normal breakdown voltages no longer conduct. Such current, however, for the abovementioned period, is not sufficient to melt the conventional solder pellet. Thereafter, when a more powerful current surge occurs, the device, if not previously replaced, does not function, and the excessive current surge is transmitted to the subscriber equipment with the distinct possibility of damage to such equipment. If the equipment should be in use at the time of the surge, the possibility of injury to the user is also present.
Consistent with this recently discovered danger, Underwriters Laboratories, has prescribed new standards wherein protection modules of this type are to fire upon the occurrence of the equivalent of 100 milliampere surge for a period of two minutes.